1. Field of the Invention
The present invention relates to a method of developing a latent electrostatic image in an electrostatic recording apparatus such as an electrophotographic reproducing apparatus. In particular, it relates to a developing method wherein an image retainer, on which a latent electrostatic image is formed, faces a developer feeding carrier of a composite developer containing a chargeable component, such as a two-component developer, across a gap between the surfaces thereof. The gap is larger than the thickness of the developer layer, and the latent electrostatic image is developed in this construction.
2. Description of the Prior Art
A composite developer is usually characterized in that the charging of toner particles which contain no magnetic substance is easy to control, because of friction with magnetic carrier particles, and accordingly are easily adsorbed in accordance with the potential of a latent electrostatic image on an image retainer. Therefore, in the method of so-called non-contact jumping development, as described above, wherein toner particles are made to jump from the body conveying and bearing the developer to the image retainer across the gap provided therebetween, the development density is usually adjusted to be constant despite any possible fluctuations in the potential of the latent electrostatic image formed on the image retainer, which are caused by changes in the density of the copy or the discharge potential of a charging electrode, etc., by a method of adjusting the diaphragm of an optical exposure system wherein the latent electrostatic image is formed by exposing an image onto the surface of an image retainer which is uniformly charged, or by a method of adjusting the intensity of a light source. These conventional methods have the problems that a complicated optical exposure system makes the apparatus expensive, and the light source employed is limited to heat-radiating types of light sources such as halogen lamps.
In the method of non-contact jumping development in which a one-component developer consisting mainly of toner particles is used, the charging of the toner particles, which usually contain a magnetic substance, is hard to control because of friction therebetween, and thus a controlling electric field is needed to provide easy control. In order to solve these problems arising in the methods of adjusting the development density as described above, methods wherein the development density was adjusted by varying the amplitude or bias of an oscillating electric field formed in a development area located in the gap between the image retainer and body conveying and bearing the developer have been proposed. Another developing method wherein the development density is adjusted by varying the frequency of an alternating voltage was disclosed in Japanese Patent Laid-Open No. 133058/1980. These methods of adjusting the development density by varying the intensity or frequency of the oscillating electric field are advantageous in that their apparatuses are less expensive than that of the method of varying the diaphragm of an optical system, and in that they are more adaptable than the method of varying the intensity of a light source. However, the method of varying the strength of the oscillating electric field can cause fogging, and the method of varying the frequency is incapable of providing a wide range of variations in development density. Anyway, it is difficult to adjust the development density by these methods to obtain a clear recorded image with fully-reproduced gradations. Furthermore, two different variable voltages are needed for changing the strength of the oscillating electric field in accordance with variations in the potentials of the latent electrostatic image and the background. This causes another problem in that a complicated power source device is needed. The method of varying the frequency requires an even more complicated power source device.